At the present time, we know of no other method to signal thru space except for EM radiation. Two problems with EM radiation. First, it's slow, compared to the wanted distances. And second, it requires very high power, AND very high power control, to control that power. Our power is but a drop, within a drop, within many more drops.....of the power needed. For transmission. For reception, the only thing that changes, IS, the EM radiation we detect from space. All the object positions in space change slowly and periodically.
We have started to play with entanglement. This appears to change state in an instant, depending on the paired state at another location. It seems to be immune to distance. And like EM, there are other problems with entanglement. Physical pairing is required.....we think. And then there is the encoding problem. After pairing, we don't know which is which. So that has to be worked on. And there might be un-forseen problems. What if the same gravity field is needed for this dynamic to function the way it does?
If there is another instant parity of this cosmos that we have not recognized, maybe aliens are using that.
But for now, EM is what we can detect. They are working on gravity detectors, but the consensus there is, that these waves travel at c also. Not to mention, how hard it would be to make a gravity wave.
If there is another communication channel in this cosmos, everybody would love to know what it is.